1. Field of the Invention
The invention is in the field of electronic solid state device processing, more particularly, measurement of minority carrier lifetime in semiconductor material.
2. Description of the Prior Art
The ability rapidly and accurately to determine various parameters of semiconductor material prior to and during processing is of critical importance in solid state device manufacturing. One of the parameters of interest is the lifetime of minority carriers in a particular sample of material. Determination of this lifetime is important in the classification of semiconductor substrate materials prior to processing, as well as to the monitoring of dopant diffusion, of ion implantation, and of the effects of other manufacturing steps.
A widely used method of semiconductor lifetime measurement is that of photoconductive decay. In this method, one illuminates the sample with optical radiation of frequency equal to or higher than the frequency required to excite electrons from the valence band across the band gap into the conduction band, thereby generating holeelectron pairs in the sample. The presence of these additional carriers results in increased conductivity of the sample. It is this increment of conductivity that is referred to as photoconductivity. After abruptly switching off the light source, the sample's conductivity relaxes back exponentially to its equilibrium value, with a time constant that is equal to the effective carrier lifetime. By measuring, with suitable apparatus, the conductivity of the sample during this relaxation one can thus determine the lifetime. The conductivity can be measured by actually applying contacts to the sample, but it can also be measured in a contactless way. Such contactless photoconductive decay methods are often applied to the determination of lifetime in large semiconductor single crystal ingots, but are generally not applied to small samples such as wafers, since typically their sensitivity is insufficient to yield an acceptable signal-to-noise ratio there.
Existing methods of lifetime determination are generally not well adapted to production line monitoring of semiconductor wafers, where by "wafer" we mean a thin disk, such as is currently generally used in device manufacture. This is particularly evident for methods that require contacting the sample. But even existing contactless methods have drawbacks that make their application for production line monitoring difficult. For instance, methods based on the observation of the relaxation of the conductivity require both light sources and measurement systems whose response is very fast compared to the sample lifetimes. This poses difficult problems for carrier lifetimes of much less than .about.1 .mu.s. Measurement techniques are reviewed in, for instance, J. Mort and D. M. A. Tai, Photoconductivity and Related Phenomena, Elsevier, 1976, and G. L. Miller et al., "Non-Destructive Electrical Test Methods for Semiconductor Materials," Proceedings of the Topical Conference on Characterization Techniques for Semiconductor Materials and Devices; Electrochemical Society, Electronics Division, V78-3, pp. 1-17 (1978).